The impellers of a counter-blow hammer are each driven by an independent air cylinder. The successful operation of the counter-blow hammer requires that the dies collide at a constant plane of impact intermediate the impellers. However, since the impellers are essentially free bodies, driven by independent air cylinders through independent pistons, controlled by independent inlet and exhaust valves, and sliding on independent ways lubricated independently, slight variations in the condition or operation of any of these structural components will tend to cause "off center" impact. The need to synchronize the impellers to assure precisely constant center of impact has been the subject of considerable study and design. Since the Impacter was first conceived, over a period of many years. Chambersburg Engineering Company has devised various ways for keeping the impellers in synchronism. An early method employed pneumatic techniques for modifying operating times for the pneumatic inlet and exhaust valves to correct errors as shown in U.S. Pat. No. 2,615,306 to Alcorn. An improved sensing means for control of such a device is shown in Alcorn U.S. Pat. No. 2,636,350. Mechanical sensing of impeller position, including differential sensing of errors in relation to impeller positions, leading to adjustments through correcting valves in the pneumatic systems of each of the raws was shown in Clarke and Weyer U.S. 2,729,943, and the modification in Clarke and Weyer U.S. Pat. No. 3,048,019. It will be noted that the techiques in each of these crases is correction directed through suitable valve means to modify the flow of supply air for driving the rams. While these systems have been effective and made the use of counter-blow hammers practical for many years, they have left room for performance improvement. Each of these prior art correction systems has also been complex, adding to the initial expense of the structure and the cost of maintenance.